1. Field of the Invention
The present invention relates to a digital signal receiver including an antenna device tunable over a wide range of frequencies.
2. Description of the Related Art
An antenna device that contains radiation conductors and variable capacitance elements coupled to the radiation conductors and that is tunable over a wide range of frequencies is available (for example, see Japanese Unexamined Patent Application Publication No. 2004-328285).
FIG. 4 is a conceptual diagram showing an antenna device described in Japanese Unexamined Patent Application Publication No. 2004-328285. On a rectangular column-shaped base member 21 made of a dielectric material or a magnetic material, for example, six band-shaped radiation conductors 22a to 22f are divided and wound. The radiation conductors 22a to 22f and varactor diodes 23a to 23f, which are variable capacitance elements and whose number is equal to the number of radiation conductors, are disposed alternately and connected in series.
FIG. 5 is an equivalent circuit diagram of the antenna device shown in FIG. 4. The radiation conductor 22a is connected between the varactor diodes 23a and 23b, and the radiation conductor 22b is connected between the varactor diodes 23b and 23c. A similar connection relationship is continued. At the end, the varactor diode 23f is connected between the radiation conductors 22e and 22f. As shown in FIG. 5, the varactors are arranged such that adjacent varactors have opposite polarities. The radiation conductor 22a is connected between anodes of the varactor diodes 23a and 23b, which are located on a feed end side. The radiation conductor 22b is connected between cathodes of the varactor diodes 23b and 23c. The radiation conductor 22c is connected between anodes of the varactor diodes 23c and 23d. The radiation conductor 22d is connected between cathodes of the varactor diodes 23d and 23e. The radiation conductor 22e is connected between anodes of the varactor diodes 23e and 23f. The radiation conductor 22f connected to the cathode of the varactor diode 23f serves as a discharge end. The above-mentioned varactor diodes 23 are provided on a side of the base member 21.
In addition, on the side of the base member 21, first electrodes 24a to 24c for grounding the anodes of the varactor diodes 23 and second electrodes 25a to 25d for applying tuning voltages to the cathodes of the varactor diodes are formed. Substantially central parts of the radiation conductors 22a, 22c, and 22e are connected to the first electrodes 24a, 24b, and 24c through resistors 26a, 26b, and 26c, respectively. A substantially central part of the cathode of the varactor diode 23a and substantially central parts of the radiation conductors 22b, 22d, and 22f are connected to the second electrodes 25a to 25d through resistors 27a, 27b, 27c, and 27d, respectively. The above-mentioned resistors 26 and 27 are also provided on the side of the base member 21.
A third electrode 28 for supplying a signal and a first electrode 24d for grounding are also formed on the side of the base member 21. The cathode of the varactor diode 23a, which is located on the feed end side, is connected to the third electrode 28 though an impedance-matching circuit 30. The impedance-matching circuit 30 includes an inductance element 30a, which is connected between the cathode of the varactor diode 23a and the third electrode 28, and a capacitance element 30b, which is connected between the third electrode 28 and the first electrode 24d. The inductance element 30a and the capacitance element 30b are also provided on the side of the base member 21. The inductance element 30a is not necessarily provided. If the inductance element 30a is not provided, the cathode of the varactor diode located on the feed end side can be directly connected to the third electrode 28.
FIG. 6 is a schematic block diagram showing a receiver including an antenna device 200 configured as described above. An RF signal output from the antenna device 200 is input to a digital tuner unit 201. The digital tuner unit 201 performs frequency conversion of the RF signal, and tunes the signal into a reception channel. The digital tuner unit 201 inputs a digital reception signal in the reception channel to a demodulation circuit 202. The demodulation circuit 202 demodulates the digital reception signal and supplies demodulated data to an application processor 203 provided at the subsequent stage. The demodulation circuit 202 also generates a tuning voltage Vtune corresponding to a reception channel designated by an external device, and applies the tuning voltage Vtune to the second electrodes 25a to 25d of the antenna device 200.
Resonance frequency characteristics and SWR characteristics of the antenna device 200 configured as described above are shown in FIGS. 7A and 7B, respectively. As shown in FIG. 7A, the resonance frequency of the antenna device 200 increases as the tuning voltage Vtune applied to the second electrodes 25a to 25d increases. In addition, as shown in FIG. 7B, the antenna device 200 exhibits narrow band-pass characteristics at resonance frequencies (fl to fh), and the narrow band-pass characteristics are shifted by the tuning voltage Vtune.
When wideband terrestrial digital broadcasting (in Japan, a broadcasting band covers a range between 470 MHz and 770 MHz) is received, if a broadcasting band (a range between 470 MHz and 770 MHz) is divided into a plurality of resonance frequencies (narrow band-pass characteristics) and received, a wide range of frequencies can be received at an antenna having a relatively small size. For example, the resonance frequency fl of the antenna device 200 when the minimum tuning voltage, Vtune=Vl, is applied is set to the lowest frequency (470 MHz) of the broadcasting band, the resonance frequency fh of the antenna device 200 when the maximum tuning voltage, Vtune=Vh, is applied is set to the highest frequency (770 MHz) of the broadcasting band, and tuning voltages applied for achieving resonance frequencies of respective reception channels and the respective reception channels have a one-to-one relationship. When a broadcasting signal is received, the demodulation circuit 202 generates a tuning voltage Vtune corresponding to a reception channel, and applies the generated tuning voltage Vtune as a preset voltage to the second electrodes 25a to 25d. The antenna device 200 has a resonance frequency corresponding to the tuning voltage Vtune, thus being capable of receiving a reception signal in the reception channel.
However, in the above-mentioned receiver of the related art, since a tuning voltage Vtune applied for a reception channel from the demodulation circuit 202 to the antenna device 200 is a fixed voltage set in advance, when a resonance frequency is shifted due to a change in usage or environment of the antenna device 200, reception quality is deteriorated.
FIG. 8A shows a state in which an actual resonance frequency is shifted from a desired resonance frequency fl when a tuning voltage, Vtune=Vl, is applied. For example, when the resonance frequency is shifted toward lower frequencies from the desired resonance frequency fl, the signal level of a demodulated reception signal exhibits a shape in which the right shoulder drops, as shown in FIG. 8B. If the signal level of a reception signal is distorted and exhibits a shape in which the right shoulder drops or the left shoulder drops, a problem may occur in that a playback image played back by the application processor 203 on the basis of the reception signal is stopped or playback sound is distorted.